this is the combo im reffering to :
0. hold down BOBBING and WEAVING
1. MAGNUM PUNCH
2. REVOLVING CANNON (-1 ammo)
3. DOUBLE BLAST
4. REVOLVING CANNON (-1 ammo)
5. release BOBBING (+2 ammo) to cancel DOUBLE BLAST animation
6. JUMP to cancel BOBBING and allow input immediately
7. hold down BOBBING
8. MAGNUM PUNCH
9. REVOLVING CANNON (-1 ammo)
10. DOUBLE BLAST
11. REVOLVING CANNON (-1 ammo)
12. release WEAVING (+2 ammo) to cancel DOUBLE BLAST animation
13. JUMP to cancel WEAVING and allow input immediately
14. hold down WEAVING
(go to step 1)
This is all done very quickly (~1.5 times per second) and it helps to have a macro for magnum punch + revolving cannon, and a second macro for double blast + revolving cannon.
The reason both bobbing and weaving are used is to cancel out the small left and right movements AND if for example only bobbing were used, the small delay before it is considered "charged" would limit the speed of the combo, thus you need to release weaving to give bobbing time to charge and vice versa. If you want to practice slowly it is fine to use only bobbing (steps 0 to 7 then repeat).